1. Field of the Invention
This invention concerns the reaction of toluene and a methylating agent in the presence of a crystalline silica catalyst, with or without promoters, to produce para-xylene.
2. Description of the Prior Art
Aluminosilicate zeolites are known as catalysts for the methylation of toluene. U.S. Pat. No. 3,965,208, U.S. Pat. No. 4,100,215, U.S. Pat. No. 4,127,616, and Yashima et al, "Alkylation On Synthetic Zeolites", Journal of Catalysis, 16, 273 and 280 (1970), are representative of the art which describes zeolites as catalysts for toluene methylation.
It is known from OS No. 2,755,770 that all or part of the alumina in an aluminosilicate zeolite can be replaced by iron oxide or by iron oxide in combination with gallium oxide. The OS states that the described metallosilicates are useful as catalysts in the methylation of toluene. The OS, therefore, broadens the store of knowledge concerning zeolites useful in methylation of toluene by suggesting the utility of iron-containing metallosilicates.
The interrelationship of the alumina (or alumina substitute) and silica content of zeolites relevant to their utility in the toluene methylation process has apparently not been studied. However, U.S. Pat. No. 3,894,103 explores the silica/alumina ratio relevant to the use of zeolites as catalysts in converting lower aliphatic alcohols such as methanol to aromatic compounds. From the Table bridging columns 5 and 6 of U.S. Pat. No. 3,894,103 it is clear that the amount of aromatic product produced decreases as the silica/alumina ratio is increased. At a ratio of 35/1 the amount of aromatic product produced is 77% or 79%; see columns 1 and 2 of the Table. At a ratio of 1300/1 no aromatic product is produced; see column 10 of the Table. The data presented in this patent would lead one to conclude that high ratios of silica to alumina are generally detrimental to catalytic activity. Thus, one would not expect very much activity of crystalline silica in the catalysis of a toluene methylation reaction.
Several crystalline silica compositions, both with and without promoters, are known. See, for instance: U.S. Pat. No. 3,941,871, U.S. Pat. No. 4,061,724, U.S. Pat. No. 4,073,865, U.S. Pat. No. 4,104,294, and Flanigen et al, "Silicalite, A New Hydrophobic Crystalline Silica Molecular Sieve", Nature, Vol. 271, 9 February 1978, pages 512 to 516.
Only in U.S. Pat. No. 4,104,294, however, is there any intimation that crystalline silica compositions might be useful as catalysts. The statement that such compositions are suitable for use in "hydrocarbon conversion" reactions is nonspecific, unexplained, and unsupported. Therefore, the patent does little to propel one skilled in the art to use crystalline silica as a catalyst since the weight of authority is that such a material would, at best, be a poor catalyst. The statement concerning the extensive area of "hydrocarbon conversion" does nothing to teach the art that such catalysts would have utility in toluene methylation or, in fact, in any specific transformation process.
Against the background of the prior art as described heretofore, it has now been found that crystalline silica containing substantially no alumina or alumina-substitute in the crystal lattice is an excellent catalyst for the specific process of methylating toluene to produce xylene, which process is characterized by particularly high proportions of the para isomer.